# A Bar Bet You Can Win… And A Little Atmospheric Science

Stuck in an airplane for five hours a few days ago, I pondered the world just outside the window. Earthlings are fragile flowers. You think space travel is harsh? We can’t even survive at 38,000 feet!

I took a look at some early morning readings from LAX. At 38,000 feet (my flight’s cruising altitude) the temperature was around -45° Celsius, or -49° Fahrenheit.

Please, don’t ask for the wind chill reading. Least it to say, survival is short at those temperatures.

Just as important, maybe more, at that altitude the atmosphere is approximately 20% as dense as it is on the ground. Each breath of outside air would only provide a fifth as much oxygen as we usually get. That short supply would quickly lead to hypoxia, then death, which is why flight attendants show oxygen masks before every takeoff!

Obviously something must be done to make the airplane livable.

My plane’s engines were providing thrust and powering compressors to pack more outside air into a smaller space. What was piped into the cabin was more to our body’s liking. Most planes don’t pressurize the air to sea level pressure, but they get reasonably close and they mix outside air with recycled and filtered cabin air.

-BAR BET ALERT-

OK, you’ve gotten this far. I might as well give you a gift… a bar bet you can win!

If it’s -49° outside the plane you would think the air has to be heated before it gets into the cabin. Nope! The air in an airplane is actually run through an air conditioner to cool it!

Through a couple of laws of thermodynamics (which I had to learn, but you don’t) pressurizing air heats it. The airplane’s compressed air is hot enough it actually needs to be chilled before it’s blasted through the vents.

So, the answer to: If it’s -49° outside your airplane, do they need a heater or air conditioner, is air conditioner!

It is customary to rip the meteorologist 10% for any bets won, but this one’s on the house.